Codes P0131 and P1133

I have a '96 Grand Am 4-cylinder that is throwing check engine codes P0131 and P1133. I haven't noticed any significant driveability issues, although I have noticed a drop in gas mileage.

I have an AllDataDIY subscription so I looked up the two codes and their diag charts. P0131 is "O2S Circuit Low Voltage Sensor 1", and P1133 is "O2S Insufficient Switching Sensor 1". I don't have a scan tool though, so I can't follow the steps listed in the diagnostic charts.

Has anyone had either or both of these codes before? If so, what was the problem when you had them? Is it likely that I just have a bad O2 sensor and that's what's throwing both codes? I just replaced the front sensor (the one referred to for both codes) last summer, so it shouldn't be bad yet, but I suppose anything is possible.

What brand of O2 did you install last year?

Check and make sure the plug on the O2 is secure and tight.
A loose connection could throw the dtc's.

Also follow the wires from the plug-in and check that thiers no breaks/shorts.

Hope that helps.